Field of the Invention
The present invention relates to an optical member exhibiting excellent optical characteristics and abrasion resistance and a method for manufacturing the optical member.
Description of the Related Art
Formation of an antireflection film, which has a thickness of several tens to several hundreds of nanometers and which is composed of a single layer or a plurality of layers of optical films stacked having different refractive indices, to suppress reflection at a light incidence and emission interface of an optical device has been known previously. Vacuum film formation methods, e.g., vapor deposition and sputtering, and wet film formation methods, e.g., dip coating and spin coating, are used for forming the antireflection films.
It is known that inorganic materials, e.g., silica, magnesium fluoride, and calcium fluoride, and organic materials, e.g., silicone resins and amorphous fluororesins, which are transparent materials having low refractive indices, are employed as the material used for an outermost layer of the antireflection film.
In recent years, it has been known that a low-refractive-index film through the use of the refractive index of the air of 1.0 is used for the antireflection film in order to further reduce the refractive index. The refractive index can be reduced by forming gaps in the layers of silica and magnesium fluoride. For example, in the case where 30 percent by volume of gaps are disposed in a magnesium fluoride thin film having a refractive index of 1.38, the refractive index can be reduced to 1.27. However, there is an issue that the abrasion resistance of the low-refractive-index layer including gaps is degraded.
Japanese Patent Laid-Open No. 2000-284102 (hereafter referred to as “Patent Literature 1”) discloses an antireflection film in which an overcoat layer containing a fluoropolymer is disposed on a low-refractive-index layer in order to improve the abrasion resistance.
Japanese Patent Laid-Open No. 2004-258267 (hereafter referred to as “Patent Literature 2”) discloses an antireflection film in which a second binder for improving the abrasion resistance is filled into gaps between hollow fine particles in a low-refractive-index layer in which hollow particles are bonded with a first binder.
However, Patent Literature 1 has an issue that a coating liquid for forming the overcoat layer enters the gaps in the low-refractive-index layer and, thereby, the refractive index of the low-refractive-index layer increases.
Meanwhile, Patent Literature 2 has an issue that the second binder is filled into the gaps between the hollow fine particles and, thereby, the refractive index increases as compared with the refractive index of an antireflection film not filled with the second binder.